A typical ultrasonic transducer used for diagnostic medical imaging commonly includes a layer of piezoelectric material, such as lead zirconate titanate (PZT), one or more acoustic impedance matching layers bonded to one side of the PZT layer and a block of backing material bonded to the other side. The backing block is a substrate of material having an arbitrary thickness. Instead of providing material as the backing block, it is possible to use an air backing.
The matching layers serve to increase the coupling of ultrasonic energy to and from the body or object to be imaged.
The transducer may be divided into an array of multiple independent small transducers (called transducer elements) to facilitate scanning of the ultrasound beam by electronic means. FIG. 1 shows a part of one transducer element 100 of such a transducer.
The backing block 102 and matching layers 104 usually have acoustic impedances lower than that of the piezoelectric layer 106, so that the piezoelectric layer 106 vibrates in a half-wave resonance mode, setting the center frequency of operation of the transducer at approximately
      f    hw    =            2      ⁢      v        d  where fhw, is the half-wave resonant frequency, v is the speed of sound in the piezoelectric material, and d is the thickness of the piezoelectric material.
Alternatively, one can design the transducer to have a backing material of higher acoustic impedance than the piezoelectric material. In this case, the transducer will operate at a center frequency approximately equal to the quarter-wave resonant frequency, fqw, given by
      f    qw    =            4      ⁢      v        d  It is clear that for a given piezoelectric material and thickness, these frequencies differ by a factor of two.
Two-dimensional (2-D) ultrasonic phased array transducers present unusual considerations in the design of the backing block. Generally, 2-D arrays require the connection of thousands of individual acoustic transducer elements to the ultrasound system electronics.
In prior art transducers, it has been recognized that it is advantageous to incorporate electronic circuitry in a handle of the transducer to provide transmit, receive, pre-amplification, and partial beam-forming functions. Connections between the acoustic elements and the electronic circuitry are made via conductors or conductive paths embedded in the acoustic backing block of the transducer (as shown for example in U.S. Pat. No. 5,592,730), or via a thin multi-layer interconnect structure between the acoustic elements and the backing block which includes conductors (as shown for example in U.S. Pat. No. 5,977,691). In either case, the electronic circuits are arranged outside of the acoustically active area of the transducer.
These prior art transducers are difficult to fabricate in view of the need to manufacture the backing block with embedded conductors and because each of the thousands of conductors must be connected to the electronic circuitry. Moreover, the presence of the interconnect structure in the active area of the transducer can result in unwanted acoustic scattering sites producing artifacts in the image. Further, the capacitance between signal traces in the interconnect structure introduces undesirable loading on the electronic circuitry and the transducer elements and provides multiple crosstalk paths between individual elements, both of which reduce the transducer's performance. The method of embedding conductors in the backing block also results in a backing block that is bulky and heavy, making the transducer difficult to use. The bulk of such a transducer also precludes the use of this method for endocavity transducers and other transducers which are used in small spaces.
With reference to FIG. 2, an alternative to a transducer with embedded conductors is a transducer 108 in which the required electronic circuitry is placed on one or more semiconductor chips 110 adjacent or in close proximity to the acoustic structure of the transducer whereby the chip with electronic circuitry typically is in the form of an integrated circuit. As a result, the interconnect structure 112 between the chip 110 and the acoustic elements 104,106 becomes nearly inconsequential electrically. Examples of this arrangement are described in U.S. Pat. Nos. 5,435,313 and 5,744,898 and U.S. provisional patent application Ser. No. 60/432,536 filed Dec. 11, 2002 entitled “Miniaturized Ultrasonic Transducer” to Sudol et al. the disclosures of which are incorporated herein by reference.
In these disclosures, acoustic effects of the electronic circuitry are ignored or an attempt is made to suppress them, as for example, by the use of a “mismatching layer” between the piezoelectric element 106 and the electronic circuit on the chip 110. However, these approaches do not yield satisfactory performance for state of the art ultrasonic imaging systems. For use with these imaging systems, a transducer is required to operate over a large bandwidth and the transmit pulses it generates must be as short as possible. For imaging of fine details for example, it is desirable to have a transmit pulse length of less than about 1.6 periods of the transmit frequency measured at about −10 dB.
In another mode called harmonic imaging, the transducer transmits ultrasound at one frequency and receives echoes at the second harmonic or twice that frequency. This requires a transducer with a one-way bandwidth of at least about 67% of the center frequency, measured at about −3 dB. In general, the minimum achievable pulse length is inversely proportional to the bandwidth. For increased performance, transducers with bandwidth approaching and even exceeding 100% of the center frequency are desired. Achieving this level of performance requires careful design of the entire acoustic structure.
The design of matching layer structures for ultrasonic transducers is well known in the art and will not be discussed in detail herein. For high performance transducers, similar attention must be paid to the backing block and any backing layers. Most often, a homogeneous composition of materials is used for the backing block to provide a uniform acoustic impedance and a high acoustic loss so as to remove the effects of reflections from the boundaries of the backing block or any internal structures that may be necessary for mechanical or thermal considerations.
The presence of an electronic chip and possibly an electrical interconnect layer between the backing block and the piezoelectric layer transforms the acoustic impedance presented to the back side of the piezoelectric layer and this transformation is dependent on the frequency. For a single backing layer, there are two sets of frequencies of particular interest. At frequencies where the backing layer is an integral number of half wavelengths thick, the acoustic impedance seen at the front of the backing layer is equal to the acoustic impedance loading the back side of the backing layer; the transformation ratio is unity (1). At frequencies where the backing layer is an odd number of quarter wavelengths thick, the acoustic impedance seen at the front of the layer is:
      Z    qw    =            Z      c      2              Z      L      where Zqw is the transformed impedance seen at the front of the backing layer, Zc is the characteristic acoustic impedance of the material of the backing layer (backing layer impedance), and ZL is the acoustic load impedance at the back side of the backing layer.
If the backing layer impedance is high compared to ZL, then the transformed impedance is much higher than the backing layer impedance itself. Conversely, if the backing layer impedance is low compared to ZL, then the transformed impedance is much lower than the backing layer impedance. In between quarter and half wave frequencies, the transformed impedance takes on values that are complex numbers with a magnitude intermediate between the backing layer impedance and the quarter wave transformed impedance.
Each backing layer further transforms the impedance generated by the backing layers behind it (when multiple backing layers are present). Since the other backing layers generate an impedance that varies with frequency, the behavior can be quite complicated, but can be modeled by the well-known transformation:
      Z          i      ⁢                          ⁢      n        =            Z      c        ⁢                            Z          L                +                  j          ⁢                                          ⁢                      Z            c                    ⁢                      tan            ⁡                          (                                                2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  d                                λ                            )                                                            Z          c                +                  j          ⁢                                          ⁢                      Z            L                    ⁢                      tan            ⁡                          (                                                2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  d                                λ                            )                                          where Zin, is the transformed impedance, 8 is the wavelength of sound in the layer material, d is the thickness of the backing layer, and j is the square root of −1. Generally, impedances lower than the backing layer impedance are transformed to high impedances and impedances higher than the backing layer impedance are transformed to low impedances.
The backing block impedance as transformed by a single layer can vary greatly with frequency and adding further layers may cause further variation resulting in large resonant peaks and nulls in the final transformed impedance. An electronic circuit, for example a silicon integrated circuit, and an associated interconnect layer will transform the impedance of the backing block behind them as just described so that at some frequencies, the transformed impedance may be very high while at others it may be very low.
As noted above, a transducer with a high backing block impedance will operate in quarter wave mode and at approximately twice the frequency of a transducer with the same piezoelectric layer but with a low impedance backing layer. A transducer with a backing block impedance that is a function of frequency may operate in quarter wave mode at frequencies where the transformed backing block impedance is high and in half wave mode at frequencies where the transformed backing block impedance is low. A transducer designed for one mode will operate poorly in the other mode, so having different modes at different frequencies within the desired operating band will result in a badly shaped, narrow spectrum possibly having resonant peaks or nulls.
Even if a mixture of modes does not occur, a frequency-dependent backing block impedance can introduce unwanted distortions into the transmitted spectrum. Such a spectrum precludes operation at multiple or harmonic frequencies and results in an unacceptably long transmit pulse.
From these considerations, it can be seen that prior art transducers incorporating an electronic circuit in close proximity to the acoustically active layers will fail to yield optimum performance.